Once you have established your time based on the high values, what happens to the high values when increasing the filter grade to darken the low values.
I didn't mention in my first post, but after the initial test strip trials (I do 3 sec strips) many times I can't really determine if, say, the 4th or 5th strip is what I want so I make another "test strip" using a full size sheet of paper starting with four 3 sec exposures (total of 12 secs), then resetting the timer for 1 sec exposures, and continuing with strips up to 15 secs. At this point, I have a full sheet of test strips with exposures of 12, 13, 14, and 15 secs; 1 sec difference in exposure can result in a visible difference in your high print values. Occasionally, I even narrow things down to 1/2 sec exposures! But, I'm crazy!!
Ian G - I've always advocated that you should be kind to the mole in the darkroom when out working in the field!